1. Field of the Invention
The present invention relates to an electroconductive composition mainly composed of copper powders, and also comprising a glass powder, as well as to a printed circuit board using the composition.
2. Description of the Related Art
At present, a thick-film printing technique is being made increasingly practical for making a printed circuit board such as a hybrid IC, wherein an electroconductive paste mainly composed of a copper powder or a silver powder is applied to an insulating board made of glass, ceramic or the like, by a screen printing method or a direct drawing method, followed by baking in order to form the desired circuit pattern (thick-film conductor).
The electroconductive pastes can be classified according to the baking temperature as a high-temperature baking type which can be baked at a temperature in the range of from about 800xc2x0 C. to about 950xc2x0 C., or as a medium-temperature baking type which can be baked at a temperature in the range of not more than 750xc2x0 C., or particularly in the vicinity of 600xc2x0 C.
While electroconductive pastes belonging to the high-temperature baking type make it possible to form a circuit pattern excellent in conductor characteristics (particularly in electroconductivity) and in adhesion to a board, they need high temperature baking, and therefore may sometimes cause thermal damage to printed resistors, dielectrics, etc. which have been formed beforehand. In comparison, those belonging to the medium-temperature baking type can be baked at a relatively low temperature, and therefore have an advantage that a circuit pattern can be formed while hardly causing thermal damages to the printed resistors, dielectrics, etc., which have been formed beforehand. However, they are somewhat inferior to those belonging to the high-temperature baking type in conductor characteristics and in bonding strength to the board.
In general, an electroconductive paste belonging to the medium-temperature baking type has a metal powder and a glass powder dispersed in an organic vehicle, and forms a sintered metal body by sintering the metal powder at the time of baking. Also, the glass powder turns to be in the liquid state in the course of baking, improving the sintering characteristics of the metal and providing the function of bonding this thick-film conductor to a board. The organic vehicle acts as a liquid medium to regulate the viscosity of the electroconductive paste so as to make these powders printable.
The adhesion of the glass powder to a board by a glass bonding mechanism in which the glass powder melts and moves to the interface between the paste and the board so as to bond the sintered metal body to the board in the course of baking the electroconductive paste. Thus, after the baking, the upper layer part of the thick-film conductor is richer in the metal component and the lower layer part is richer in the glass component, with the result that the glass component is in a state in which it is likened to arms stretching from the surface of the board up through the metal particles, and plays a role of mechanically bonding the board and the thick-film conductor.
Electroconductive pastes mainly composed of a copper powder have been made increasingly practical in recent years. This is because the copper powder has a small wiring resistance and is excellent in migration characteristics, in addition to the fact that it is less expensive in comparison with a silver powder or silver-palladium powders.
It is necessary to maintain the baking of electroconductive pastes mainly composed of a copper powder in a non-oxidizing atmosphere in which the oxygen content is not more than 5 ppm since the copper powder is susceptible to oxidation. However, since it is believed in general that the wetting of a copper powder with a glass occurs with the aid of oxygen, the oxygen content of not more than 5 ppm at the time of baking lowers the wettability of the copper powder with the molten glass, allowing the glass to hinder the sintering of the copper particles, with the result that the strength and the electroconductivity of the thick-film conductor itself after the sintering are lowered sometimes.
The present invention was achieved in consideration of the above-described circumstances, and one of the objects according to the present invention is to provide an electroconductive composition having an excellent electroconductivity and insuring sufficiently large bonding strength to a board and also to provide a printed circuit board on which a circuit pattern is formed, using this electroconductive composition.
Accordingly, one aspect according to the present invention is an electroconductive composition comprising copper powders and a lead-free glass powder, the copper powders comprising (1) from about 50% to 90% by weight of a copper powder A containing from about 1% to 3% by weight of oxygen, and having an average particle size of from about 0.9 to 1.5 xcexcm; and (2) from about 10% to 50% by weight of a copper powder B containing from about 0.2% to 3% by weight of oxygen and having an average particle size of not more than about 0.6 xcexcm.
When the electroconductive composition according to the present invention is used, the wettability between the copper powders and the glass is improved by virtue of the specific amounts of oxygen existing in the copper powders A and B, so that the liquid-phase sintering of the copper powders by the glass is smoothly facilitated, the thick-film conductor thus obtained becomes dense, and the electroconductivity and the bonding strength to the board are increased to a great extent. At the same time, since the copper powder B with a relatively small average particle size is contained at a specific rate, the thick-film conductor can be made sufficiently denser, and its electroconductivity and bonding strength are further improved.
It is noted that when the oxygen content in the copper powder A is less than about 1% by weight, a sufficient bonding strength cannot be obtained since the liquid-phase sintering of the copper powders by the glass does not proceed smoothly, while the conductor resistance becomes larger and the soldering properties are degraded when it exceeds about 3% by weight. Furthermore, when the average particle size of the copper powder A is less than about 0.9 xcexcm, the bulk density becomes smaller, requiring a larger amount of an organic vehicle to form a paste. On the other hand, when the average particle size exceeds about 1.5 xcexcm, the sintering properties are degraded.
Also, it is noted that a sufficient bonding strength cannot be obtained when the oxygen content in the copper powder B is less than about 0.2% by weight. This is because the liquid-phase sintering of the copper powders by the glass does not proceed smoothly, as is the case of the copper powder A. On the other hand, the conductor resistance becomes larger, when it exceeds about 3% by weight. Furthermore, when the average particle size of the copper powder B exceeds about 0.6 xcexcm, a thick-film conductor which is dense and has a large bonding strength cannot be obtained since the packing efficiency is not improved sufficiently by mixing the particles of the copper powders A and B.
Furthermore, when the rate of the copper powder A is less than about 50% by weight in the copper powders consisting of the copper powders A and B (in other words, when the rate of the copper powder B exceeds about 50% by weight), the bulk density of the whole copper powder becomes smaller. On the other hand, when the rate of the copper powder A exceeds about 90% by weight (in other words, when the rate of the copper powder B is less than about 10% by weight), the packing efficiency is not improved sufficiently by mixing the particles of the copper powders A and B, thus failing to provide a thick-film conductor with a large bonding strength.
The oxygen content in the copper powder A or B according to the present invention means the amount of the oxygen in the respective entire copper powder expressed in % by weight. The content can be increased, for example, by keeping the powder standing in an oven at about 150xc2x0 C. for a specific time period, and it can be decreased by subjecting the powder to a reduction treatment in a hydrogen atmosphere. The oxygen content in the copper powder can be analyzed, for example, by an inert gas fusion-infrared ray absorption method or the like as described in Japanese Industrial Standard (JIS) H1067.
Also, the present invention provides a printed circuit board with a specific circuit pattern provided thereon wherein at least part of the circuit pattern is formed with an electroconductive composition according to the present invention.
According to the present invention, a printed circuit board having a circuit pattern with an excellent electroconductivity, as well as with a sufficiently large bonding strength to the board, can be obtained since the circuit pattern is formed with the electroconductive composition according to the present invention. Furthermore, since the electroconductive composition according to the present invention can be baked at a relatively low temperature, a printed circuit board having various good properties as well as excellent reliability can be obtained without providing thermal damage to other circuit elements existing on and in the printed circuit board.